A variety of swimming pool cleaning devices have been designed in the past for automatically travelling over and cleaning the bottom and side surfaces of a swimming pool. This type of cleaner operates on the reduced pressure (suction) caused by a pump which induces a flow of water from the pool through a filtration system and back into the pool. Some of these devices travel under water by producing a pulsatile interruption of the flow of water through the cleaner using a special valve assembly. For example, Australian Patent No. 502,993 describes an underwater suction cleaner having a balanced operating head with an oscillatable valve adapted to close a pair of flow passages extending through the head from the inlet to an outlet. The valve is of substantially triangular cross-section so that it can pivot about its apex adjacent the outlet. A baffle plate causes one of the flow passages to be more restricted and less direct between the inlet and the outlet than the other. The valve is shaped so that liquid flowing through the passages causes automatic oscillation of the valve between first and second positions in which one of the flow passages through the head is closed.
Most of these prior art cleaning devices have not proved to be entirely satisfactory due to either too many moving parts or an imbalance in the basic design of the device. These problems can result in premature break-down, excessive wear of certain parts, entanglement of the vacuum hose or immobilisation of the device when it encounters a corner or other obstruction in the pool.